Wide beam CT dosimetry. Elly Castellano

Transcription

1 Wide beam CT dosimetry Elly Castellano

2 Outline revision: CT dose indices wide-beam CT: the end of the road for CTDI? the IEC rescue plan for CTDI 100 the american way AAPM report 111 better estimates of patient dose AAPM report 204 effective dose calculations options for wide beam CT 2

3 Revision: CT dose indices 3

4 Multiple Scan Average Dose axial scanning beam rotation + translation beam width T = increment NT for MSCT MSAD = average dose in centre of irradiated volume tends towards equilibrium value MSAD Shope et al

5 CT Dose Index MSAD can be measured instead on one axial scan CTDI = CT dose index CTDI 1 T D z d or NT for MSCT z CTDI = MSAD for equivalent scan range Shope et al

6 CTDI in practice measured parallel to axis of scanner using pencil ionisation chamber 100 mm integration length CTDI 100 free-in-air, or in dose phantoms in terms of air kerma 6

7 CTDI in practice measured parallel to axis of scanner using pencil ionisation chamber 100 mm integration length CTDI 100 free-in-air, or in dose phantoms in terms of air kerma dose (mgy) distance (mm) 7

8 CTDI 100 in dose phantoms cylindrical PMMA phantoms with holes for pencil chamber 32 cm body phantom 16 cm head phantom 14 cm depth CTDI 100 measured at centre and 1 cm below surface 8

9 CTDI 100 in dose phantoms cylindrical PMMA phantoms with holes for pencil chamber 32 cm body phantom 16 cm head phantom 14 cm depth CTDI 100 measured at centre and 1 cm below surface IAEA HHR #5 9

10 CT dose descriptors based on CTDI 100 combined with actual scan parameters to indicate dose to patient CTDI w weighted value of central & peripheral values in PMMA phantom 1 2 CTDI w CTDI 100, c CTDI 100, 3 3 linear increase in dose along radius assumed p 10

11 CT dose descriptors CTDI vol takes account of helical pitch or axial scan increment CTDI w CTDI vol p DLP takes into account scan range DLP CTDI vol R CTDI vol and DLP displayed on scanner console 11

12 CTDI mgy Limitations of CTDI 100 underestimates MSAD for scan ranges > 100 mm typical scan range mm overestimates MSAD for scan ranges < 100 mm defined for axial scanning only overestimates MSAD for stationary scans extension to helical scanning presumptuous scan range mm 12

13 Limitations of CT dose descriptors CTDI w defined empirically CTDI vol inaccurate under AEC DLP good indicator of integral dose Dixon and Boone 2013 they are not patient dose phantoms not representative of human body 100 mm unrepresentative of clinical scan ranges 13

14 Wide beam CT: the end of the road for CTDI? 14

15 Wide-beam CT MSCT 40 mm (Siemens) 80 mm (Philips, GE) 160 mm (Toshiba) CBCT interventional units linac on-board imaging NM localisation imaging 15

16 D(z) and increasing beam width free-in-air Mori et al

17 D(z) and increasing beam width centre of 900 mm long body phantom Mori et al

18 D(z) and increasing beam width periphery of 900 mm long body phantom Mori et al

19 D(z) and increasing beam width free-in-air dose profile widens with collimation heel effect becomes evident in phantom dose profile widens with collimation D(0) increases with collimation tends to equilibrium value analogous to train of contiguous narrow beam profiles 19

20 CTDI 100 and increasing beam width free-in-air well-defined <100 mm definition breaks down for NT 100 mm 20

21 CTDI 100 and increasing beam in long phantom stable 40 mm decreases for mm definition breaks down for 80 mm diverging primary beam wider than 100 mm width CTDI 100 efficiency = CTDI 100 CTDI Boone

22 The IEC rescue plan for CTDI

23 IEC Edition 3 two definitions of CTDI 100 : choice of denominator NT for NT < 100 mm 100 mm for NT > 100 mm 23

24 IEC Edition 3 for beams < 100 mm no change for beams > 100mm measure average dose over 100 mm CTDI 300 for 160 mm beam (Geleijns et al 2009) but different CTDI efficiency CTDI mgy scan range mm 24

25 IEC Edition 3 for NT 40 mm Amendment 1 for NT > 40 mm NT ref 20 mm integration length for CTDI free-in-air max{nt+40 mm, 100 mm} 25

26 IEC Edition 3 Amendment 1 no change for beams 40 mm for beams > 40mm same CTDI efficiency body phantom IAEA HHR #5 26

27 Why so much effort to hang on to CTDI 100? test equipment available 100 mm pencil chambers 140 mm head and body phantoms practicalities transporting 300 mm PMMA phantoms no consensus on new phantom length 27

28 IEC Edition 3 Amendment 1: in practice modality MSCT CBCT - DR CBCT - RT CBCT - NM availability of NT ref range of collimations available in axial mode collimation set manually Varian user choice Elekta 20 mm collimator narrow collimation available in service mode 28

29 IEC Edition 3 Amendment 1: in practice CTDI free-in-air CT chamber centred well beyond table to reduce scatter use 100 mm table feed to step chamber through beam 160 mm beam IAEA HHR #5 29

30 The american way: AAPM report

31 Rationale for report 111 CTDI has been outgrown defined for axial scanning only helical and CBCT geometries now ubiquitous CTDI 100 breaks down for wide beams 31

32 Rationale for report 111 new CT metrics for acceptance testing and QC axial, helical and stationary scanning all beam widths and scan lengths uniform phantoms of sufficient length new CT dose descriptors for patient dose estimates 32

33 Glossary f(z) single-scan dose profile nt nominal beam width a collimation width half f(0) b table increment per rotation b= helically L scan range along z-axis L=Nb axially L= t helically AAPM report

34 Cumulative dose in phantoms for scans with table translation 34

35 Cumulative dose cumulative dose profile D L (z) smoothed for general applicability Shope et al 1981 AAPM report

36 Cumulative dose: axial scanning oscillatory with period b smoothed by averaging over z ± b/2 at each value of z convolution with rectangular function Π (z/l) 1 L/2 L/2 z 36

37 Cumulative dose: helical scanning no smoothing required along central axis non-oscillatory function for right cylindrical phantoms smoothed by angular averaging over 2 at each value of z equivalent to smoothing along z axis 37

38 Cumulative dose: helical scanning 38

39 Cumulative dose at the scan general expression: midpoint at scan midpoint: at midpoint for L= 39

40 CT metric # 1: equilibrium dose D eq equilibrium reached at L eq ~ 400 mm for this example measurements practical D eq a/b dose distribution broadens for L > L eq no scatter reaches z=0 40

41 CT metric # 2: equilibrium dosepitch product where p = pitch independent of p, b can be measured at any convenient p =b/nt equal to CTDI 41

42 CT metric # 3: equilibrium dose constant independent of a, b equilibrium dose when increment = beam width complete specification of the midpoint dose and can be calculated for any other beam widths note nt/a is 1/geometric efficiency (tabulated) 42

43 Cumulative dose in phantoms for stationary scans 43

44 Cumulative dose for N rotations: at scan midpoint: analogous measurement to D L (0) unlikely to reach D eq beam widths too narrow 44

45 Dose free-in-air 45

46 Dose free-in-air expressed in terms of equilibrium dosepitch product: f air (z) dose profile free-in-air for single axial rotation equal to CTDI 46

47 CT dose descriptors 47

48 Integral dose total energy absorbed in phantom where f(r,z) axial dose profile at radius r from central axis R phantom radius phantom density 48

49 Planar average equilibrium dose denoted by related to E tot by valid for any scanning length L 49

50 Comments on CT dose descriptors E tot is not the energy deposited in the scanned volume is not the average dose over the scanned volume 50

51 AAPM report 111 in practice 51

52 Test equipment phantoms uniform sufficiently long > 450 mm shape, size and composition not yet specified 30 cm water phantom, 50 cm long 32 cm PMMA phantom, 45 cm long 52

53 Test equipment detectors thimble chamber mm active length volume > 0.6 cm 3 other point dosimeters TLDs solid state detector RadCal 0.6 cm 3 chamber 53

54 Measurement technique with translation chamber in midpoint along chosen axis centre, 1 cm below surface typical select reference scan protocol kvp, mas, focus beam-shaping filter beam width table increment / pitch < ½ chamber length average out oscillations 54

55 Measurement technique with L eq is unknown translation so measure approach-to-equilibrium function h(l) for selection of scan ranges L beware of helical overranging! 55

56 Measurement technique with fit function of form* translation evaluate D eq and L eq calculate equilibrium dose-pitch product calculate equilibrium dose constant if a is known * for 32 cm PPM phantom, Dixon and Ballard

57 Measurement technique with translation equilibrium dose-pitch product for other collimations can be calculated if a / nt is known or measured at L L eq repeat all measurements for other kvps, beam-shaping filters repeat along other axes repeat with other phantoms 57

58 Measurement technique without translation chamber in midpoint along chosen axis centre, 1 cm below surface typical select reference scan protocol kvp, mas, focus beam-shaping filter beam width measure f(0) directly reference scan protocol other permutations 58

59 Measurement technique free-inair chamber centred free-in-air, clear of table select reference scan protocol measure dose integral translate chamber through beam calculate equilibrium dose-pitch product if a/nt is known, calculate for other collimations, otherwise measure measure for other permutations 59

60 Why are we not embracing AAPM report 111? equipment not available too heavy to carry fillable water phantoms pose an electrical hazard extensive measurements required more suited to type testing than QC? no closer to patient dose than CTDI

61 Better estimates of patient dose: AAPM report

62 Rationale for AAPM report 204 patient dose depends on scanner radiation output patient size CTDI vol provides information only on scanner output adopting CTDI vol as patient dose can result in large underestimates e.g. factor 2-3 for paediatric scans using 32 cm dose phantom 62

63 Objectives conversion factors to estimate patient dose applied to displayed CTDI vol for subjects of all sizes user-friendly radiologists, technologists, physicists 63

64 General approach estimate patient dose from patient size size-specific conversion factor radiation output metric normalisation of conversion factors by CTDI vol eliminates variations due to different beam qualities kvp, scanner filtration, geometry etc Turner et al

65 General approach patient size described by AP dimension LAT dimension AP+LAT effective diameter (AP x LAT) patient size determined using electronic calliper on SPR or CT scan physical calliper on patient 65

66 AAPM report 204: deriving conversion factors 66

67 Review of research studies data from 4 research groups combined different materials and methods 2 phantom-based studies 2 Monte Carlo-based studies 67

68 Group 1: Mc group anthropomorphic torso phantoms 11 sizes: 9 to 39 cm LAT dimension additional scattering material superior and inferior 4 CT scanner models abdominal protocol helical, axial, cine clinical scan ranges measured D L (0) at centre and periphery calculated area mean D L (0) (1/3 C + 2/3 P) normalised by displayed CTDI vol 68

69 Group 2: TS group uniform PMMA phantoms 3 sizes: 10, 16, 32 cm diameter, 15 cm long water-equivalent diameter (WED) calculated for each all CT vendors, 18 models, range of kvps measured CTDI vol and normalised as function of WED 100 mm scan range implicit established WEDs and LAT sizes of head, chest and body of children from SPRs and CT scans produced tables of CTDI vol factors v. LAT size a 69

70 Group 3: MG group mathematical voxel phantoms 8 sizes: newborn to large adult MCNPX code 4 CT scanner models abdominal protocol 150 to 330 mm scan range calculated organ doses within irradiated volume normalised by calculated CTDI vol and averaged over all scanners plotted against patient perimeter at central slice x z y 70

71 Group 4: ZB group mathematical uniform cylindrical phantoms many sizes: 1 to 50 cm, infinitely long water, PMMA SIERRA code 1 CT scanner model, range of kvps 10, 100 mm and scan range interpolate to estimate 200 to 300 mm scan range calculated dose to water at centre and periphery calculated mean (1/3 C + 2/3 P) x z y normalised by calculated CTDI vol 71

72 Comparison of data 120 kvp coefficients normalised to 32 cm phantom good agreement 10 cm spectral differences? good agreement with Huda 2000 thoracic study similar result for 16 cm phantom 72

73 Comparison of data coefficients for range of kvps normalised to 32 cm phantom data from 2 groups R 2 =0.973 for single best fit within 5.1% of 120 kvp data 73

74 Outcomes 120 kvp coefficients adopted considered most robust data look-up tables generated conversion factor v. AP, LAT, AP+LAT, effective diameter 32 cm dose phantom 74

75 AAPM report 204 in practice 75

76 User-friendly dose estimate estimate of mean dose at the central slice of a clinically realistic scan range f defined for patient size indicator X, 32 cm phantom X = A for AP, L for LAT, D for effective diameter equivalent expression for 16 cm phantom 76

77 User-friendly dose estimate obtain CTDI vol for scan series confirm reference dose phantom determine patient dimension LAT from SPR beware of miscentering AP and LAT from CT scan beware of FOV LAT from direct measurement effective diameter from age select and calculate SSDE from ICRU 74 77

78 Will SSDE catch on? IEC exploring mandatory implementation patient size measured automatically SSDE displayed with CTDI vol and DLP risk of drifting towards constant dose scan protocols lower doses required to image smaller patients weight kg SSDE for equal noise mgy 78

79 Effective dose calculations: options for wide beam CT 79

80 Available CT dose calculators Monte Carlo source data phantoms normalisation measurable quantities NRPB R-248 to R ImPACT dose calculator adult male MIRD, fixed weight CTDI ICRUmuscle free-in-air CTDI air free-inair CTDI air free-in-air GSF CT conversion factors + CT-Expo adult male and female MIRD, Child and Baby, fixed weight CTDI air free-inair CTDI w CTDI w 80

81 ImPACT dose calculator courtesy: ImPACT 81

82 CT-Expo courtesy: Stamm 82

83 Effective dose estimates for wide beam CT ImPACT dose calculator can be used for full rotation scans partial rotation scans with random start adopt IEC edition 3 amendment 1 experimental methods reference collimation to match scanner to available MC data set new integration limits to measure CTDI-in-air 83

84 Effective dose estimates for wide beam CT PCXMC20Rotation could be used for full rotation scans partial rotation scans with random or fixed start x-ray beam modelled by adding beams of varying sizes x-ray beam quality parameters required air kerma at isocentre, rather than CTDI, to calculate organ doses 84

85 PCXMC20Rotation courtesy: Stamm 85

86 Primary references Status of Computed Tomography Dosimetry for Wide Cone Beam Scanners, IAEA Human Health report 5, 2011 Comprehensive Methodology for the Evaluation of Radiation Dose in X-Ray Computed Tomography, AAPM report 111, 2010 Size-specific Dose Estimates (SSDE) in Pediatric and Adult Body CT Examinations, AAPM report 204,

87 Secondary references 87